[SRU][Jammy][lowlatency] Ubuntu: [Config] lowlatency: enhance desktop responsiveness
Gerald Yang
gerald.yang at canonical.com
Tue Oct 31 13:48:55 UTC 2023
Hi Dimitri,
Thanks for checking this!
And yes, that would be very helpful to enable it in focal lowlatency-hwe
and jammy lowlatency-hwe kernels,
at least users/customers could have a way to use this feature.
Another question I'd like to ask is roughly when will 6.5 lowlatency-hwe
kernel be landed in Jammy? Should I also submit this to 6.2 lowlatency
kernel?
Thanks,
Gerald
On Tue, Oct 31, 2023 at 8:36 PM Dimitri John Ledkov <
dimitri.ledkov at canonical.com> wrote:
> Hi,
>
> On Tue, 31 Oct 2023 at 10:54, Gerald Yang <gerald.yang at canonical.com>
> wrote:
> >
> > Hi Andrea,
> >
> > Thanks for the feedback! please let me explain this in more detail
> >
> > I sent out this SRU because some of our customers need to use NO_HZ_FULL.
> > In their scenario, the CPU time is pretty sensitive, and must avoid as
> much noise as possible to keep
> > most CPUs doing only one single task.
> > I found some CPU steal time happen on their VM because RCU callback
> softirqs are still triggered on CPUs
> > with isolcpus, nohz_full and rcu_nocbs configured, and lead me to find
> NO_HZ_FULL is not built into generic kernel.
> >
> > I think the use case for NO_HZ_FULL should be different than generic
> kernel, e.g. performance vs low latency
> >
> > On generic kernel, we also did some tests with NO_HZ_FULL built-in here:
> > https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1919154
> >
> > The results show performance degradation on AMD EPYC series machines only
> > when NO_HZ_FULL is built-in but not enabled, but this doesn't happen on
> Intel and arm64 machines.
> >
> > Because one of the customer is still on Focal, if it's possible to merge
> this into 5.15 kernel, it would be more convenient
> > for them to use HWE lowlatency kernel.
> > And if there are users need to use NO_HZ_FULL on Focal, lowlatency
> kernel could be a proper choice for them,
> > because so far there is no kernel with NO_HZ_FULL built-in on Focal.
>
> I am ok having this enabled in focal:lowlatency-hwe-5.15 and
> jammy:lowlatency-hwe-6.5
>
> but not change anything in focal:lowlatency (5.4 / ga)
> jammy:lowlatency (5.15 / ga).
>
> Would that help you? As an exception to our policy. I really feel
> uncomfortable changing default GA flavours, but risk levels of making
> hwe kernels more like the future sounds safer.
>
> >
> > Thanks,
> > Gerald
> >
> > On Tue, Oct 31, 2023 at 2:28 PM Andrea Righi <andrea.righi at canonical.com>
> wrote:
> >>
> >> On Tue, Oct 31, 2023 at 12:57:39AM +0800, Gerald Yang wrote:
> >> > BugLink: https://bugs.launchpad.net/bugs/2023007
> >> >
> >> > [Impact]
> >> >
> >> > The lowlatency kernel in Ubuntu is specifically designed to prioritize
> >> > high responsiveness, making it ideal for multimedia environments like
> >> > DAWs and audio processing platforms, as well as soft real-time
> >> > environments.
> >> >
> >> > With the introduction of a real-time kernel, it might be worth
> >> > reconsidering the role of the lowlatency kernel and potentially
> >> > including it as the default kernel in desktop images, focusing on its
> >> > suitability for desktop-oriented usage.
> >> >
> >> > To achieve this, we can enable additional configuration settings and
> >> > make it more focused for a low-latency and highly responsive desktop
> >> > environment.
> >> >
> >> > Optionally (for the future) provide also an additional user-space
> >> > package that would enable specific run-time kernel settings focused at
> >> > certain preset workload profiles (e.g, web navigation, gaming, audio
> >> > processing, etc.).
> >> >
> >> > [Test case]
> >> >
> >> > Use linux-lowlatency in a desktop environment and measure
> responsiveness
> >> > of interactive applications.
> >> >
> >> > [Fix]
> >> >
> >> > Enable the following additional .config settings to make this kernel
> >> > more suitable for a low-latency desktop kernel:
> >> >
> >> > - CONFIG_NO_HZ_FULL=y: enable access to "Full tickless mode"
> (shutdown
> >> > clock tick when possible across all the enabled CPUs if they are
> >> > either idle or running 1 task - reduce kernel jitter of running
> tasks
> >> > due to the periodic clock tick, must be enabled at boot time
> passing
> >> > `nohz_full=<cpu_list>`)
> >> >
> >> > - CONFIG_RCU_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y: move RCU
> >> > callbacks from softirq context to kthread context (reduce time
> spent
> >> > in softirqs with preemption disabled to improve the overall system
> >> > responsiveness, at the cost of introducing a potential performance
> >> > penalty, because RCU callbacks are not processed by kernel threads)
> >> >
> >> > [Regression potential]
> >> >
> >> > Enabling all these settings can introduce a potential performance
> >> > regression, but the kernel should result more responsive and make
> >> > it more suitable for a desktop/multimedia/gaming/audio processing
> >> > context.
> >> >
> >> > Signed-off-by: Gerald Yang <gerald.yang at canonical.com>
> >>
> >> It's worth mentioning that we already applied these configs to the
> >> lowlatency kernels in mantic and in general I don't see any downside if
> >> we apply this.
> >>
> >> However, I'm not sure if a change like this is proper SRU material, we
> >> are basically saying "now lowlatency is a desktop kernel everywhere".
> >>
> >> Certain workloads may experience regressions (especially with the
> >> RCU_NOCB_CPU), while NO_HZ_FULL is a lot safer (since it can be
> >> enabled/disabled via boot options).
> >>
> >> We also need to keep in mind that Jammy will get these changes with the
> >> lowlatency-hwe-6.5 kernel, so I'm not still 100% convinced to backport
> >> this and apply it everywhere... opinions?
> >>
> >> -Andrea
> >>
> >> > ---
> >> > debian.lowlatency/config/annotations | 13 +++++++++++++
> >> > 1 file changed, 13 insertions(+)
> >> >
> >> > diff --git a/debian.lowlatency/config/annotations
> b/debian.lowlatency/config/annotations
> >> > index 694ae26b6745..3e1556d60809 100644
> >> > --- a/debian.lowlatency/config/annotations
> >> > +++ b/debian.lowlatency/config/annotations
> >> > @@ -15,12 +15,20 @@ CONFIG_HZ_250
> note<'Override default HZ used i
> >> > CONFIG_LATENCYTOP policy<{'amd64':
> 'y', 'arm64': 'y'}>
> >> > CONFIG_LATENCYTOP note<'
> https://lists.ubuntu.com/archives/kernel-team/2014-July/045006.html,
> LP#1655986'>
> >> >
> >> > +CONFIG_NO_HZ_FULL policy<{'amd64':
> 'y', 'arm64': 'y'}>
> >> > +CONFIG_NO_HZ_FULL note<'Enable access
> to "Full tickless mode" (LP: #2023007)'>
> >> > +
> >> > +CONFIG_NO_HZ_IDLE policy<{'amd64':
> 'n', 'arm64': 'n'}>
> >> > +CONFIG_NO_HZ_IDLE note<'Disabled in
> favor of CONFIG_NO_HZ_FULL (LP: #2023007)'>
> >> > +
> >> > CONFIG_PREEMPT policy<{'amd64':
> 'y', 'arm64': 'y'}>
> >> > CONFIG_PREEMPT note<'Enable fully
> preemptible kernel'>
> >> >
> >> > CONFIG_PREEMPT_VOLUNTARY policy<{'amd64':
> 'n', 'arm64': 'n'}>
> >> > CONFIG_PREEMPT_VOLUNTARY note<'Disable
> voluntary preemption model'>
> >> >
> >> > +CONFIG_RCU_NOCB_CPU policy<{'amd64':
> 'y', 'arm64': 'y'}>
> >> > +CONFIG_RCU_NOCB_CPU note<'Move RCU
> callbacks from softirq context to kthread context (LP: #2023007)'>
> >> >
> >> > # ---- Annotations without notes ----
> >> >
> >> > @@ -55,6 +63,8 @@ CONFIG_CEC_PIN
> policy<{'amd64': 'y', 'arm64': '
> >> > CONFIG_CEC_PIN_ERROR_INJ policy<{'amd64':
> 'n', 'arm64': 'n'}>
> >> > CONFIG_COMEDI_TESTS_EXAMPLE policy<{'amd64':
> 'n', 'arm64': 'm'}>
> >> > CONFIG_COMEDI_TESTS_NI_ROUTES policy<{'amd64':
> 'n', 'arm64': 'm'}>
> >> > +CONFIG_CONTEXT_TRACKING policy<{'amd64':
> 'y', 'arm64': 'y'}>
> >> > +CONFIG_CONTEXT_TRACKING_FORCE policy<{'amd64':
> 'n', 'arm64': 'n'}>
> >> > CONFIG_DEBUG_PREEMPT policy<{'amd64':
> 'n', 'arm64': 'n'}>
> >> > CONFIG_HZ policy<{'amd64':
> '1000', 'arm64': '1000'}>
> >> > CONFIG_INLINE_READ_LOCK policy<{'arm64':
> '-'}>
> >> > @@ -89,4 +99,7 @@ CONFIG_PREEMPT_RCU
> policy<{'amd64': 'y', 'arm64': '
> >> > CONFIG_PREEMPT_TRACER policy<{'amd64':
> 'n', 'arm64': 'n'}>
> >> > CONFIG_TASKS_RCU policy<{'amd64':
> 'y', 'arm64': 'y'}>
> >> > CONFIG_TEST_DIV64 policy<{'amd64':
> 'm', 'arm64': 'm'}>
> >> > +CONFIG_TICK_CPU_ACCOUNTING policy<{'amd64':
> '-', 'arm64': '-'}>
> >> > CONFIG_UNINLINE_SPIN_UNLOCK policy<{'amd64':
> 'y', 'arm64': 'y'}>
> >> > +CONFIG_VIRT_CPU_ACCOUNTING policy<{'amd64':
> 'y', 'arm64': 'y'}>
> >> > +CONFIG_VIRT_CPU_ACCOUNTING_GEN policy<{'amd64':
> 'y', 'arm64': 'y'}>
> >> > --
> >> > 2.25.1
> >> >
> >> >
> >> > --
> >> > kernel-team mailing list
> >> > kernel-team at lists.ubuntu.com
> >> > https://lists.ubuntu.com/mailman/listinfo/kernel-team
> >
> > --
> > kernel-team mailing list
> > kernel-team at lists.ubuntu.com
> > https://lists.ubuntu.com/mailman/listinfo/kernel-team
>
>
>
> --
> okurrr,
>
> Dimitri
>
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